In the books
In the A Song of Ice and Fire novels, Styr is one of Mance Rayder's lieutenants and the Magnar - "Lord" in the Old Tongue of the First Men - of the Thenn people, a clan of Free Folk established in a valley north of the Frostfangs. In the series, much of his plotline from A Storm of Swords is taken by Tormund Giantsbane - including being mistaken by Jon Snow for Mance, and leading the climb over the Wall.
When Jon first enters Mance Rayder's tent, Styr is one the six people inside. He is a tall lean man with no ears, clean shaven, bald with a straight nose and grey eyes.
- Styr at a Wiki of Ice and Fire (MAJOR spoilers from the books)